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1、LasertechnologyR.E.SlusherBellLaboratories,LucentTechnologies,MurrayHill,NewJersey07974Lasertechnologyduringthe20thcenturyisreviewedemphasizingthelasersevolutionfromsciencetotechnologyandsubsequentcontributionsoflasertechnologytoscience.Asthecenturydrawstoaclose,lasersaremakingstrongcontributionstoc
2、ommunications,materialsprocessing,datastorage,imagerecording,medicine,anddefense.Examplesfromtheseareasdemonstratethestunningimpactoflaserlightonoursociety.Laseradvancesarehelpingtogeneratenewscienceasillustratedbyseveralexamplesinphysicsandbiology.Free-electronlasersusedformaterialsprocessingandlas
3、eracceleratorsaredescribedasdevelopinglasertechnologiesforthenextcentury.S0034-6861(99)02802-01. INTRODUCTIONLighthasalwaysplayedacentralroleinthestudyofphysics,chemistry,andbiology.Lightiskeytoboththeevolutionoftheuniverseandtotheevolutionoflifeonearth.Thiscenturyanewformoflight,laserlight,hasbeend
4、iscoveredonoursmallplanetandisalreadyfacilitatingaglobalinformationtransformationaswellasprovidingimportantcontributionstomedicine,industrialmaterialprocessing,datastorage,printing,anddefense.Thisreviewwilltracethedevelopmentsinscienceandtechnologythatledtotheinventionofthelaserandgiveafewexamplesof
5、howlasersarecontributingtobothtechnologicalapplicationsandprogressinbasicscience.Therearemanyotherexcellentsourcesthatcovervariousaspectsofthelasersandlasertechnologyincludingarticlesfromthe25thanniversaryofthelaser(AusubellandLangford,1987)andtextbooks(e.g.,Siegman,1986;AgrawalandDutta,1993;andRead
6、y,1997).Lightamplificationbystimulatedemissionofradiation(LASER)isachievedbyexcitingtheelectronic,vibrational,rotational,orcooperativemodesofamaterialintoanonequilibriumstatesothatphotonspropagatingthroughthesystemareamplifiedcoherentlybystimulatedemission.Excitationofthisopticalgainmediumcanbeaccom
7、plishedbyusingopticalradiation,electricalcurrentanddischarges,orchemicalreactions.Theamplifyingmediumisplacedinanopticalresonatorstructure,forexamplebetweentwohighreflectivitymirrorsinaFabry-Perotinterferometerconfiguration.Whenthegaininphotonnumberforanopticalmodeofthecavityresonatorexceedsthecavit
8、yloss,aswellaslossfromnonradiativeandabsorptionprocesses,thecoherentstateamplitudeofthemodeincreasestoalevelwherethemeanphotonnumberinthemodeislargerthanone.Atpumplevelsabovethisthresholdcondition,thesystemislasingandstimulatedemissiondominatesspontaneousemission.Alaserbeamistypicallycoupledoutofthe
9、resonatorbyapartiallytransmittingmirror.Thewonderfullyusefulpropertiesoflaserradiationincludespatialcoherence,narrowspectralemission,highpower,andwell-definedspatialmodessothatthebeamcanbefocusedtoadiffraction-limitedspotsizeinordertoachieveveryhighintensity.Thehighefficiencyoflaserlightgenerationis
10、importantinmanyapplicationsthatrequirelowpowerinputandaminimumofheatgeneration.Whenacoherentstatelaserbeamisdetectedusingphoton-countingtechniques,thephotoncountdistributionintimeisPoissonian.Forexample,anaudiooutputfromahighefficiencyphotomultiplierdetectingalaserfieldsoundslikeraininasteadydownpou
11、r.Thislasernoisecanbemodifiedinspecialcases,e.g.,byconstantcurrentpumpingofadiodelasertoobtainasqueezednumberstatewherethedetectedphotonssoundmorelikeamachinegunthanrain.Anopticalamplifierisachievedifthegainmediumisnotinaresonantcavity.Opticalamplifierscanachieveveryhighgainandlownoise.Infacttheypre
12、sentlyhavenoisefigureswithinafewdBofthe3dBquantumnoiselimitforaphase-insensitivelinearamplifier,i.e.,theyaddlittlemorethanafactoroftwotothenoisepowerofaninputsignal.Opticalparametricamplifiers(OPAs),wheresignalgainisachievedbynonlinearcouplingofapumpfieldwithsignalmodes,canbeconfiguredtoaddlessthan3
13、dBofnoisetoaninputsignal.InanOPAthenoiseaddedtotheinputsignalcanbedominatedbypumpnoiseandthenoisecontributedbyalaserpumpbeamcanbenegligiblysmallcomparedtothelargeamplitudeofthepumpfield.2. HISTORYEinstein(1917)providedthefirstessentialideaforthelaser,stimulatedemission.Whywasntthelaserinventedearlie
14、rinthecentury?Muchoftheearlyworkonstimulatedemissionconcentratesonsystemsnearequilibrium,andthelaserisahighlynonequilibriumsystem.Inretrospectthelasercouldeasilyhavebeenconceivedanddemonstratedusingagasdischargeduringtheperiodofintensespectroscopicstudiesfrom1925to1940.However,ittookthemicrowavetech
15、nologydevelopedduringWorldWarIItocreatetheatmosphereforthelaserconcept.CharlesTownesandhisgroupatColumbiaconceivedthemaser(microwaveamplificationbystimulatedemissionofradiation)idea,basedontheirbackgroundinmicrowavetechnologyandtheirinterestinhigh-resolutionmicrowavespectroscopy.Similarmaserideasevo
16、lvedinMoscow(BasovandProkhorov,1954)andattheUniversityofMaryland(Weber,1953).ThefirstexperimentallydemonstratedmaseratColumbiaUniversity(Gordonetal.,1954,1955)wasbasedonanammoniamolecularbeam.Bloembergensideasforgaininthreelevelsystemsresultedinthefirstpracticalmaseramplifiersintherubysystem.ThesedeviceshavenoisefiguresveryclosetothequantumlimitandwereusedbyPenziasandWilsoninthediscoveryofthecosmicbackgroundradiation.Towneswasconfidentthatthemaserconceptcouldbeextendedto
